Issues in Window Selection: Energy-related
Heat Transfer Mechanisms | Measuring Properties | Overview of Energy Use | Codes and Standards
Measuring Energy-related Properties
There are four energy performance characteristics of windows used to portray how energy is transferred and are the basis for how energy performance is quantified. They are:
- U-factor. When there is a temperature difference between inside and outside, heat is lost or gained through the window frame and glazing by the combined effects of conduction, convection, and radiation. The U-factor of a window assembly represents its insulating value.
- Solar Heat Gain Coefficient. Regardless of outside temperature, heat can be gained through windows by direct or indirect solar radiation. The ability to control this heat gain through windows is measured in terms of the solar heat gain coefficient (SHGC) or shading coefficient (SC) of the window.
- Visible Transmittance. Visible transmittance (VT) is an optical property that indicates the amount of visible light transmitted through the glass (also referred to as visible light transmittance - VLT). It affects energy by providing daylight that creates the opportunity to reduce electric lighting and cooling loads.
- Air Leakage. Heat loss and gain also occur by air leakage through cracks in the window assembly. This effect is measured in terms of the amount of air (cubic feet or cubic meters per minute) that passes through a unit area of window (square foot or square meter) under given pressure conditions. In reality, infiltration varies slightly with wind-driven and temperature-driven pressure changes. Air leakage also contributes to summer cooling loads by raising the interior humidity level.
Insulating Value (U-factor)
Heat flow from the warmer side to the colder side of a window and frame is a complex interaction of all three basic heat transfer mechanisms--conduction, convection, and radiation. The ability of the window assembly to resist this heat transfer is referred to as its insulating value. Heat flows from warmer to cooler bodies, thus from inside to outside in winter, and reverses direction in summer during periods when the outside temperature is greater than indoors. Conduction occurs directly through the glazing material itself as well as through the solid parts of the spacers and frames.
Determining Insulating Value
The U-factor (also referred to as U-value) is the standard way to quantify insulating value. It indicates the rate of heat flow through the window. The U-factor is the total heat transfer coefficient of the window system (in Btu/hr-sq ft-°F or W/sq m-°C), which includes conductive, convective, and radiative heat transfer. It therefore represents the heat flow per hour (in Btus per hour or Watts) through each square foot (or square meter) of window for a 1°F (1°C) temperature difference between the indoor and outdoor air temperature. The R-value is the reciprocal of the total U-factor (R=1/U). As opposed to an R-value, the smaller the U-factor of a material, the lower the rate of heat flow.
Overall U-factor
Since the U-factors are different for the glass, edge-of-glass zone, and frame, it can be misleading to compare U-factors if they are not carefully described. In order to address this problem, the concept of a total window U-factor is utilized by the National Fenestration Rating Council (NFRC). A specific set of engineering assumptions and procedures must be followed to calculate the overall U-factor of a window unit using the NFRC method. Originally developed for manufactured window units, there is a new method for certifying site-built assemblies. The whole unit U-factor will vary, of course, depending on the glass-to-frame ratio. Figure 2-7 indicates the center-of-glass U-factor and the overall U-factor for several types of window units. Since Windows A-G in Figure 2-7 have a thermally broken aluminum frame, the center of glass U-factor is always lower than the whole unit. The whole window and center-of-glass U-factors for Windows H and I are closer because of the insulated frames.
Solar Radiation Control
The second major energy-performance characteristic of windows is the ability to control solar heat gain through the glazing. Solar heat gain through windows is a significant factor in determining the cooling load of many commercial buildings. The origin of solar heat gain is the direct and diffuse radiation coming from the sun and the sky or reflected from the ground and other surfaces. Some radiation is directly transmitted through the glazing to the space, and some may be absorbed in the glazing and then indirectly admitted to the space. Other thermal (nonsolar) heat transfer effects are included in the U-factor of the window. Sunlight is composed of electromagnetic radiation of many wavelengths, ranging from short-wave invisible ultraviolet, to the visible spectrum, to the longer, invisible near-infrared waves. About half of the sun's energy is visible light; the remainder is largely infrared with a small amount of ultraviolet. While reducing solar radiation through windows is a benefit in some climates and during some seasons, maximizing solar heat gain can be an energy benefit under winter conditions.
Determining Solar Heat Gain
There are two means of indicating the amount of solar radiation that passes through a window. These are solar heat gain coefficient (SHGC) and shading coefficient (SC). In both cases, the solar heat gain is the combination of directly transmitted radiation and the inward-flowing portion of absorbed radiation (Figure 2-9). However, SHGC and SC have a different basis for comparison.
Shading Coefficient
Until recently, the shading coefficient (SC) was the primary term used to characterize the solar control properties of glass in windows. Although it is being replaced by the solar heat gain coefficient (SHGC), it is still referenced in books and product literature.
The shading coefficient (SC) is only defined for the glazing portion of the window and does not include frame effects. It represents the ratio of solar heat gain through the system relative to that through 1/8-inch (3 mm) clear glass at normal incidence. The shading coefficient is expressed as a dimensionless number from 0 to 1. A high shading coefficient means high solar gain, while a low shading coefficient means low solar gain.
For any glazing, the SHGC is always lower than the SC. To perform an approximate conversion from SC to SHGC, multiplying the SC value by 0.87.
Solar Heat Gain Coefficient
Window standards are now moving away from use of shading coefficient to solar heat gain coefficient (SHGC), which is defined as that fraction of incident solar radiation that actually enters a building through the window assembly as heat gain. The SHGC is influenced by all the same factors as the SC, but since it can be applied to the entire window assembly, the SHGC is also affected by shading from the frame as well as the ratio of glazing and frame. The solar heat gain coefficient is expressed as a dimensionless number from 0 to 1. A high coefficient signifies high heat gain, while a low coefficient means low heat gain. Typical SHGC values for the whole window unit and center of glass are shown in Figure 2-10.
Visible Transmittance
Visible transmittance (VT), also referred to as visible light transmittance (VLT), is the amount of light in the visible portion of the spectrum that passes through a glazing material. A higher VT means there is more daylight in a space which, if designed properly, can offset electric lighting and cooling loads due to lighting. Visible transmittance is influenced by the glazing type, the number of layers, and any coatings that might be applied to the glazings. Visible transmittance of glazings ranges from above 90 percent for water-white clear glass to less than 10 percent for highly reflective coatings on tinted glass. Typical visible transmittance values for the whole window unit and center of glass are shown in Figure 2-12.
Air Leakage (Infiltration)
Air leakage (infiltration) can be defined as ventilation that is not controlled and usually not wanted. It is the leakage of air through cracks in the building envelope. Air leakage leads to increased heating or cooling loads when the outdoor air entering the building needs to be heated or cooled. Operating windows can be responsible for a significant amount of the air leakage in buildings. Tight sealing and weatherstripping of windows, sashes, and frames is of paramount importance in controlling air leakage.
Cracks and air spaces left between the window unit and the building wall can also account for considerable infiltration. Insulating and sealing these areas during construction or renovation can be very effective in controlling air leakage. A proper installation ensures that the main air barrier of the wall construction, is effectively sealed to the window or skylight assembly so that continuity of the two air barriers is maintained.
The amount of air that leaks through all of the cracks in and around a window sash and frame is a function of crack length, tightness of the seals and joints, and the pressure differential between the inside and outside. Window manufacturers currently report air leakage test values as cfm/sq ft (cubic feet per minute per square foot of window area). In the past, air leakage may have been reported as cfm/lf (cubic feet per minute per lineal foot of sash crack) but this form of reporting is no longer supported.